Homework Help:
HELP - Molar mass changes at stp?

Hey, I have this lab report due tomorrow, and I seem to be getting some discrepencies in my calculations. I am calculating the molar mass of butane, without using the values from the periodic table, using data obtained by the experiment below:

I won't bother you with error propagation as it is easy to caculate, so disregard that. Below are my calculations for the molar mass of butane, at the temp and pressure given above and at STP (standard temp and pressure).
Am I doing this properly? Do my results make sense? (Ignore the "..." they are for placement)

No, you should not be getting different answers. One mol of a quantity contains a specific number of molecules (Avogadro's number) and so has a specific mass. That doesn't change just because you change temperature and pressure (although the volume might.)

Oh I did the "easy" part in my lab notebook of course. Since i made the correction (post 2 of mine) i have values approx 61 and 64. But I think I'm using the wrong equation or something..because ivy is right, molar mass should always stay the same.